A study of the relationship between the Electromyographic (EMG) signal and the isometric force output of three human skeletal muscles will be performed on 16 healthy adults. The subjects will be organized in four categories, each consisting of approximately 4 subjects: accomplished long distance swimmers, accomplished weight lifters, outstanding concert pianists and subjects not having any particular exercise specializations. The possible dependence of the EMG-force relationship on functionally different muscles such as the first dorsal interosseous, biceps brachii and deltoid, and on different forms of exercise will be investigated. The statistical and physiological properties of single motor units during varying force and force-rate contractions will be investigated mathematically according to random-processes analysis approaches that have proven successful for isometric constant-force contractions. The purpose of this study is to a) provide a basis for relating the EMG signal to the force output of human skeletal muscles, and b) to describe some physiological properties of the motor units in normal muscles and indicate the effect of motor unit controllability as a function of specific forms of exercise. This study will also provide baseline data, which can be used to quantitatively measure the difference between the performance of normal or abnormal muscles, and thus should results in a useful clinical implement for evaluating the state of a muscle.